There are many measurement devices for optically measuring a property of a measurement object available on the market today. For example measurement devices using laser are used in various applications such as chemical analysis, spectroscopy, optical disc readers, and metrology.
One such measurement device for measuring small distances and other physical parameters which are capable of altering an optical path length is described in GB 1038140. The device comprises a He—Ne laser, an external reflector which is aligned with the laser, and a photo detector. The reflector is movable so that the phase of the reflected light returned into the laser may be adjusted in relation to the light emitted from the laser, whereby the generation of laser light is intensified or counteracted, respectively. The generation is detected by the photo detector, whereby the reading of the photo detector is periodical having the period λ/2. Thus, the movements of the external reflector, i.e. the change in optical length between the laser and the reflector, may be determined from the readings of the photo detector.
However, a problem with the measurement device disclosed in GB 1038140, as well as with other known measurement devices, is that it due to the construction is sensitive to misalignments, especially misalignment of the external reflector, and at the same time quite expensive to manufacture and difficult to control. Moreover, the reading in GB 1038140 is periodical, i.e. non-monotone, whereby it is not possible to directly obtain an absolute measurement of for example the distance.